1. Field of the Invention
The invention relates to engineering instrumentation, and more particularly it relates to time-of-flight ion mass analyzers designed for the determination of mass and isotope composition of substances used in solving general problems of chemical analysis, and more particularly for the determination of mass and isotope composition of plasma in vacuum.
2. Description of the Prior Art
Known in the art is a time-of-flight ion mass analyzer or a mass reflectron in which ions of a studied substance which are formed by a focused laser radiation or under the action of an electron beam are reflected from a reflector during their flying asunder or after preacceleration and are recorded by a detector. The initial energy of ions once ionized and the flight time being known, the ion mass can be determined. A space-time focusing of ion packets, diffusing because of a spread in the initial energy, is effected in the reflectron so that the instrument has high mass resolution (up to 3000) (cf. B.A. Mamyrin, Mass Reflectron, Jr. of Experimental and Theoretical Physics (in Russian), v. 64, No. 1, 1973).
The known instrument cannot, however, be used for the determination of ion mass if one does not know exactly the starting time so that the instrument cannot be used without its own ion source which is to inject ions into the instrument (generally during a time of a maximum 1 to 10 ns). Therefore, the above described instrument cannot be used as a mass analyzer of ion beams incoming from the outside.
Known in the art is a time-of-flight ion mass analyzer comprising a device for preacceleration of ions being studied, a sensing member of an analyzer used to detect the moment at which an ion enters the time-of-flight space (in the form of carbon foil) and an electron detector connected to a time interval measurement device, which are located in series downstream the preacceleration device (cf. "Comet Halley Neutral Gas Experiment--CHALLENGE", Proposal Submitted to ESA in Response of Giotto. Call for Experiment Proposals. Pr. SCI (80)7. Max-Planck Institut fur Aeronomie, Lindau, 1980).
In the prior art mass analyzer the mass analysis of individual ions coming from the outside is effected in accordance with their time-of-flight through a pre-set portion of the instrument, i.e. through the time-of-flight space, provided the particles have a low spread of energy and a low initial energy. Generally such ions are accelerated in the preacceleration device of the instrument to an energy of 45 to 70 keV and are then caused to pass through a thin carbon foil (about 2 .mu.g/cm.sup.2). Secondary electrons emitted from the foil, which are recorded by means of a detector in the form of a system of microchannel plates (MCP) serve as a starting signal for counting the time of ion movement through the time-of-flight space of a pre-set length. A similar MCP system at the end of the time-of-flight space is used to measure the ion arrival time and generates a STOP signal. Given the initial ion energy and their flight time, one can determine the mass of singly charged ions.
The prior art mass analyzer is, however, deficient in low mass resolution in recording heavy ions. This is due to the fact that with an increase in the ion mass the effective spread in energy losses during ion movement through the foil becomes higher. For this reason it is necessary to have high preacceleration voltage in the receiving part of the instrument.
Nevertheless, in accordance with the reference, with a preacceleration voltage in the instrument of about 75 kV for masses of about 100 amu the mass resolution M/.DELTA.M is as low as 10 (M/.DELTA.M is about 40 for M of about 40 amu) so that mass peaks of isotopes of medium and heavy substances cannot be resolved. The provision of a preacceleration system operating at high voltage in the instrument restricts the field of its application, substantially lowers its reliabiity because of liability to high-voltage breakthrough, and the instrument's structure is complex and heavy.